Modulated carrier wave



Patented Sept. 12, 1939 MODULATED [CARRIER WAVE TRANSMITTER William Stuart Leader Tringham, Puttenham, Guildford, England, assignor to Radio Corporation of America, a corporation of Delaware Application October 10, 1936, Serial No. 104,957 In Great Britain October 24, 1935 2 Claims. (Cl. 179-1715) This invention, which is for improvements in potentials which may be derived from the difor modifications of the invention contained in ferent stages of the transmitter and passed on United States application, Serial No. 753,291, filed to the cathode ray tube. November 15, 1934, by W. S. L. Tringham, which According to another feature of this invention, issued on April 13, 1937, as Patent No. 2,076,685, a rectifier provided for rectifying modulated car- 5 relates to modulated carrier wave transmitters rier wave energy taken from the transmitter and and apparatus for use therewith. More speto be fed to the switching means and thence to cifically the invention relates to what may be t e c h d ay tu is located adjacent said termed modulation monitoring arrangements switching means, i. e., at or near the control for use in connection with radio and like transdesk. 10 mitters and whereby the correctness of the mod- Th invention s illustrated in the p yulation at different stages of the transmitter may g d aw n which Figure 1 Shows diagrame Checked from time to time and during opermatically an embodiment for use in an installaation of the said transmitter. tion wherein provision is required to be made for 5 Though not exclusively limited in its app1i monitoring the output from three different stages cation thereto, the invention is primarily intendf a tr mit namely a sub-modulator e, ed for "monitoring television transmitters. a modulator stage, and a high frequency mod- According to the parent invention a modulaulated a p fi a Figures 2 d 3 Show a tion monitoring installation, for use for the pur- Preferred o m of w c which y be used in pose specified, comprises a cathode ray tube, Place O that Shown in Figure 80 means for deriving from a plurality of sta of Referring to Figure 1 there is employed as the transmitter to be monitored potentials which switching means a ary swi ch SW like that should be like functions of the degree of modillustrated in the drawing c o p ny n the ulation, and switching means or it equivalent parent specification and having twelve contacts ,3 whereby any of the potentials so derived may arranged in a circle a tWO mutually Derpe be applied to deflect the ray in the cathode ray dicular Contact arms Al, teach ad pted to t b t deflection of t ray t obtained bridge diametrically opposite contacts, said arms serving as an informative indication of the corbeing arranged to be rotated together- The fixed rectness or otherwise of operation f the transcontacts in this switch, will be designated in the tt same way as in the parent specification, i. e., they 3 It fr e tly happens that the monitoring will be numbered clockwise from I to I2, contact dication i required to be given at some point NO. taken as the Contact at 12 oclockhe ei after t h termed the t desk"v Contacts I 2, l and 2 are connected together and which is some appreciable distance f o the through a suitable bias battery GB to the input transmitting apparatus proper, the various grid G of an m l comprising a11y desired 35 transmitter and modulating controls being genof ampllfylng stages arranged to mainerally arranged at the control d k, tam the direct current component during am- It is also desirable, in carrying out the inven plification and whose output circuit contains a tion contained in the parent patent, t be able resistance across which potentials are tapped off .h) to employ as the monitoring indicator a cathode P t deflecting plates of the cathode ray tube ray tube of small sensitivity. The present inven- 'f In 1 only one Valve V is Shown, tion enables the cathode ray tube indicator to be the resistance, across which potentials are taken located at the control desk and well away f off to the deflector plates of the tube, not shown, the transmitting apparatus proper without difbeing marked In P e O u n a battery ficulties due to the wide range of frequencies re- GB any other Suitable bias source, e. g., a rectiquired to be handled, in a television transmitter r arr ngement, could, of course, be employed. in particular these frequencies may occupy a very Contacts 3, and 5 are C(mhected e er nd wide range, and furthermore facilitates the satto the cathode C of e ve A r d resistisfactory use of a cathode ray tube of low sensiance GL is connected directly between contacts tivity 2 and contact 3. Contact 6 is connected to earth 50 Accgrding t t i feature of thi m through a resistance RI and also through a suittion there is provided between the switching able high qu y inductance ChOke CHtO t means and the cathode ray tube of an installacathode CI of a diode VI whose anode Al is tion in accordance with the parent invention an connected to earth through a parallel tuned ciramplifier which serves to amplify any of the cult L2, K2, shunted byaresistance R2. Contact I is connected through a suitable cable HFC, preferably a. high frequency cable, to a variable tapping T upon the anode resistance AR of the sub-modulator stage, not shown, to be monitored. This cable of course extends from the control desk, indicated by the broken line, to the point where the rest of the transmitter is located. Contacts 8. 9, and H) are connected together and to earth. Contact H is connected through a second cable HFCI, preferably also a high frequency cable, to a variable tapping Tl upon the anode resistance ARI of the modulator stage to be monitored. Modulated high frequency out put is taken, for example as shown, via a condenser K, from the live member of the modulated output feeder F of the transmitter, not shown, and the energy thus taken is fed through a high frequency cable H'FCZ to an adjustable tapping T2 upon the inductance in the parallel tuned circuit L2 K2 in the anode circuit of the diode VI. This parallel tuned circuit is tuned to the working frequency and the resistance R2 is made sufiicient to maintain the frequency response reasonably fiat up to about 3 megacycles. Preferably the three cables used have outer screens which are earthed and the arrangement is such in each case that the frequency response obtained at the control desk ends of the feeders is reasonably fiat up to about 3 megacycles, for the case of modern high definition television.

It is obviously not essential to take off the monitoring voltages from intermediate tappings T and TI upon the anode resistance AR and ARI as illustrated and in some cases it may be found better to take off voltage from the live end of resistance AR and/or from the live" end of resistance ARI instead of using the intermediate tap method.

Where direct connection to the live end of an anode resistance is employed, suitable series resistance should, of course, be inserted in the appropriate lead by which the voltage in question is taken in order to reduce the said voltage to the level required. This method of taking off monitoring voltage is most likely to be preferable in the case of the modulator resistance ARI. It is common practice to house the modulator and modulated amplifier in different cubicles and to connect the two by means of a high frequency feeder. If this feeder is of any substantial length the frequency response of the modulation signals will be different according to which end of the feeder it is measured at. Obviously it is preferable to monitor at the input to the modulated amplifier.

It is not necessary to take the energy to the rectifier VI from the feeder F in the precise manner shown. For example, the'live member of the feeder F may be connected to one terminal of a condenser of, say 10 micro-micro-farads, whose other terminal is earthed through a low resistance of, say 2 ohms, the outer feeder conductor is usually earthed, the "live end of this resistance being taken to the rectifier. Such a circuit will give a satisfactorily level response for the monitoring system. A similar circuit may also be used to take off modulated high frequency from the feeder to a rectifier feeding into a local, monitoring, television reproducing system, not shown, so .as to enable the quality of the pictures transmitted to be observed at the monitoring station. A defect of this type of circuit wherein a capacity of about 10 micromicro-farads in series with a low resistance is shunted across the feeder is that this shunt cirone of about 61 ohms, and thus serves cuit will, unless its point of connection be carefully chosen, have a marked, undesired effect upon the input impedance of the said feeder. Where such a circuit is employed, therefore, the shunt circuit is, according to a subordinate feature of the invention, connected across the feeder at a point substantially /8 of the working wave length from the transmitter and of said feeder. When so connected the shunt circuit acts as an impedance transformer, a-capacity of 10 micro-micro-farads at this point will transform an input impedance of about 70 ohms into a dual purpose, for an impedance transformer is often required at this point in .a television transmitter system-i. e., near the transmitter end of the aerial feeder so that a certain amount of loading from the aerial may be thrown into the anode circuit of the final amplifier of the transmitter without having to couple said anode circuit so closely to the input coil of the feeder as to make the said anode circuit difficult to tune. The shunt circuit which serves the dual purpose referred to, should, of course, be as free from inductance as possible.

Figures 2 and 3 show a preferred form of rotary cam operated switch which may be substituted for the switch SW of Figure 1. Figure 2 is a schematic end view of the switch, with the operating knob removed, and Figure 3 is a schematic elevation. The operating knob O is mounted on the end of a shaft 5 on which cams CMI, CM2, CM3 of insulating material are keyed. The cams are staggered or phase displaced in a rotary sense so that, as the knob O is rotated, each cam protuberance points upwards in turn. Each cam operates two pairs of contacts of telephone type in a manner which will be obvious from the figure. The contacts in Fig. 3 are given references corresponding to those of switch SW of Fig. 1 and the switch of Fig. 3 is shown in the position corresponding to that in which the switch SW of Fig. 1 is shown. By rotating knob 0 through 120 from the position shown in Fig. 3 contacts I .and I and 4 and II will be closed and the other contacts opened.

It will be seen that with arrangements as above described a common amplifier is employed for all the energies to be fed to the cathode ray tube .and in practice the deflecting voltages available at the tube will commonly be of the order of 150 volts or more, assuming the valve to give a voltage magnification of about 3, so that with two thousand volts on the electron gun anode of the tube it will be possible with present day practice to obtain on the fluorescent screen of the tube, an image which is easily visible in daylight with a deflection of nearly 30 millimetres.

The specific embodiments above described and illustrated are suitable for a case where the A. C. and D. C. voltages with respect to earth are opposite at the modulator monitoring point as compared to the sub-modulator monitoring point. In some cases, however, the D. C. voltages may be in the same sense but the A. C. voltages in opposite sense. In such a case the above described switching arrangements could be employed but a suitable bias source should be connected in series with one monitoring cable.

Having described my invention, what I claim as new and desire to secure by Letters Patent is:

1. A modulation monitoring installation used with a modulated radio transmitter, wherein is provided a plurality of stages, comprising a cathode ray oscillograph, means for deriving from a plurality of stages of the transmitter monitoring potentials which are representative of the degree of modulation, a single rotary cam-operated switch for selecting one of the potentials so derived, a wide frequency band amplifier connected between the switch and the cathode ray oscillograph and a rectifier having a damped resonant input circuit interposed between one of the plurality of stages and the switch.

2. A modulation monitoring installation used with a modulated radio transmitter, wherein is provided a plurality of stages, comprising a cathode ray oscillograph, means for deriving from a plurality of stages of the transmitter monitoring potentials which are representative of the degree of modulation, a single rotary cam-operated switch for selecting one of the potentials so derived, a wide frequency band amplifier connected between the switch and the cathode ray oscillograph, a rectifier having a damped resonant input circuit, said rectifier being connected to said switch, a concentric cable feeder, a serially connected resistance and capacitor connected across one-eighth of the working wave length of the feeder, and connections from the serially connected resistor and capacitor to the damped input circuit of said rectifier.

WILLIAM STUART LEADER TRINGHAM. 

